Periodic reverse current pulsing to form uniformly sized feed through conductors

ABSTRACT

A large number of electrically conductive solid, dense feed-through paths for the high-speed low-loss transfer of electrical signals between integrated circuits of a single silicon-on-sapphire body, or between integrated circuits of several silicon-on-sapphire bodies, are provided by an electroforming method utilizing periodic reverse-current pulsing.

CONTRACTUAL NOTICE

The invention described herein was made in the performance of work underNASA Contract No. NAS 5-25654 and is subject to the provisions ofSection 305 of the National Aeronautics and Space Act of 1958 (72 Stat.435; 42 U.S.C. 2457).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 200,700 filedOct. 27, 1980, now abandoned.

The invention herein is related to the invention disclosed and claimedin U.S. patent application Ser. No. 285,656, filed of even date herewithin the name of inventor Anthony, and entitled "Implantation ofElectrical Feed-Through Conductors". Application Ser. No. 285,656 is acontinuation-in-part of U.S. patent application Ser. No. 200,770, filedOct. 27, 1980, which is assigned to the same assignee as the instantinvention and is now abandoned. Application Ser. No. 423,334, filedSept. 24, 1982, is a division of application Ser. No. 285,656. Both saidSer. No. 285,656 and Ser. No. 200,770 applications are hereinincorporated by reference.

The invention herein is also related to the invention disclosed andclaimed in U.S. patent application Ser. No. 204,957, filed Nov. 7, 1980in the name of inventors Anthony, Connery and Hoeschele, Jr., entitled"Method of Forming Conductors Through Silicon-on-Sapphire, and Product",which is assigned to the same assignee as the instant application, andis also incorporated herein by reference. Also related to and assignedto the same assignee as the instant invention is the invention disclosedand claimed in U.S. patent application Ser. No. 244,854, filed Mar. 18,1981, in the name of inventor Anthony which is entitled"Alignment-Enhancing Feed-Through Conductors for StackableSilicon-on-Sapphire Wafers."

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the manufacture of semiconductor devices andmore particularly to the formation of a large number of solid, denseelectrically conductive paths through semiconductor bodies in order toreduce the number and length of conductive interconnections betweenlogic and switching elements on a single body and/or between bodies in amulti-body system.

2. Description of the Prior Art

Computer science has developed in an era of computer technology in whichwire interconnects were inexpensive and logic and switching elementswere expensive. Integrated circuit technology has recently reversed thecost situation leaving wire interconnects as the more expensivecomponent. Interconnections between the integrated circuits of a singlechip or wafer, whether made of wires or strips of conducting material,are expensive because they occupy most of the space on the wafer andcause most of the delay in electronic signals passing through thesystem. The same reasoning holds for interconnections between wafers.Computer architecture theory has just begun to take the cost reversalgenerated by integrated circuit technology into consideration. As aresult, computer design has not yet taken advantage of the full range ofcapabilities implicit in microelectronics.

Current advances in computer design involve the development of amassively parallel information processing system for ultrahigh speedprocessing of multiple digital data streams. Such multiple data streamsare encountered in situations where interactions of the physical dataare significant as, for example, in image processing and studies ofweather conditions, economics, hydrodynamics and stresses. The massivelyparallel array processor with many processors operating simultaneouslyand in parallel requires many interconnections between processors. Withmultiple processors, the number of interconnections, the space occupiedby interconnections, the delay time caused by interconnections, thepower consumed in interconnections, and the cost of interconnections hasincreased as the square of the number of processors in the system.

The massively parallel array processor system is built utilizingComplementary Metal Oxide Semiconductor/Silicon-on-Sapphire Large ScaleIntegration (CMOS/SOS LSI) circuitry. Processor arrays on manyindividual silicon-on-sapphire wafers must also be interconnected. Incurrent technology, all such interconnections must run out to a pad onthe edge of a wafer or chip. Such an interconnection scheme has severaldisadvantages.

First, the number of interconnection pads on the periphery of an LSIcircuit is very limited. The relatively small number of interconnectionpads severely restricts the information flow to and from an LSI circuit.For example, a typical memory chip has 16,384 bits arranged in a 128 by128 array. An entire row of 128 bits can be assessed at one time, but aselector enables only a single bit to pass to an output pin. A typicalmemory system is made of 2,048 such chips arranged in 64 groups of 32.Only 32 chips can place their outputs on the 32 wires that join the busto the central processor. Of the 262,144 bits that move less than amillimeter on each chip, only 2,048 move 3 millimeters to get off theirchip and only 32 move a meter to the processor. In other words, becauseof an effective traffic tie-up on the interconnections, only abouteight-thousandths of the available density of the memory chip can beused at present.

The second disadvantage of the interconnection scheme used by currenttechnology is that a large fraction of the area of an LSI circuit isdevoted to interconnections. This waste of a large area of a chip or awafer is a direct consequence of the restriction of interconnections tosubstantially two-dimensional configurations. Previous methods ofproviding conventional conductive paths in three-dimensionalconfigurations by placing the paths in layers on one chip have generallyresulted in a decrease in the quality of the processed information dueprimarily to the phenomenon of cross-talk.

SUMMARY OF THE INVENTION

In accordance with the above-referenced Ser. No. 204,957 invention, alarge number of small diameter closely-spaced electrically conductivepaths are introduced through silicon-on-sapphire wafers, or chips, usedin information processing equipment. These through-wafer conductingpaths substantially reduce the number and length of conductive pathsneeded on the front face of the wafer; increase the speed and quality ofinformation processing; reduce the power consumed by and the heatgenerated in interconnections; and provide many more access paths tologic, switching and memory elements on the front face of the wafer.Further, these through-wafer conducting paths provide a means ofsubstantially reducing the physical space occupied by microelectroniccircuits by allowing the silicon-on-sapphire wafers to be stacked one onanother with the feed-through conductors in each wafer serving asinterconnection paths from wafer to wafer.

In accordance with the present invention, there is provided a noveldifferent method of introducing the electrically conductive materialinto holes extending through the thickness of the wafer. The method ofthis invention is particularly advantageous and useful when the numberof through-thickness holes is greater than about a thousand.

Briefly described, the method of this invention involves the steps ofproviding a suitable body of silicon-on-sapphire or a semiconductormaterial having a plurality of holes therethrough, positioning the bodyin a suitable electroforming apparatus opposite from an anode,surrounding the body and the anode with an electroforming solution,initiating a flow of bubbles of an inert gas in the space between theanode and the body, establishing a flow of direct current through theelectroforming solution between the body and the anode, forming a solidfilm bridge of metal across the bottoms of the holes, reversingperiodically the flow of the direct current, growing in the holes asolid, dense implant from the solid film bridge at the bottoms of theholes to the tops of the holes, and, optionally, continuing said growthto form implants having rivet-like terminations of a controlled andsubstantially equal diameter at each end adjacent to the top and bottomsurfaces of the body.

The article of the invention is a body of semiconductor material orsilicon-on-sapphire having a plurality of solid, dense metallic implantstherethrough. The density of the material of the implants is equal to orgreater than about 95% of theoretical. The implants have the shape ofright circular cylinders whose exposed ends are substantially flush oreven with the planes of the top and bottom surfaces of the body. Theimplants may optionally be terminated in rivet-like caps at oppositeends. The rivet-like caps are adjacent to and overlie the major top andbottom surfaces of the body, or any layer of material thereon, serve tolock the implants in place, and, additionally, may make contact withactive devices or connecting lines also situate on the major surfaces.The size of the rivet-like caps can be controlled so that for apopulation of one million such caps no more than three caps will deviatefrom the average diameter of the population by more than ±9%.

The diameter of the cylindrical portion of the implant is typicallyequal to or less than about 4 mils and the center line-to-center linespacing is typically less than or equal to about 2 cylindricaldiameters. The lengths of the cylindrical sections are equal to thethickness of the body which typically ranges from about 6 to about 100mils, thus these cylindrical sections have high-aspect-ratios typicallygreater than or equal to 6.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is more clearly understood from the following descriptiontaken in conjunction with the accompanying drawings wherein some detailshave been disproportionately enlarged for clarity and of which:

FIG. 1 is a dimensional cross-section of a typical silicon-on-sapphirebody.

FIG. 2 is an enlarged schematic elevation view in cross-section of asection of the silicon-on-sapphire body of FIG. 1 prepared for laserhole drilling.

FIG. 3 is a schematic dimensional view in cross-section of thesilicon-on-sapphire wafer of FIG. 2 following the laser drilling ofholes through the wafer.

FIG. 4 is a schematic top view in cross-section of thesilicon-on-sapphire body of FIG. 3 disposed in the electroformingapparatus.

FIG. 5 is a schematic elevation view in cross-section of a section of asilicon-on-sapphire wafer in which the electro-formed feed-throughconductors have rivet-like terminations.

FIG. 6 is a schematic elevation view in cross-section of a section of asilicon-on-sapphire body showing one implant terminated in a rivet-likehead which makes contact with two circuit elements on the silicon-facedsurface of the body.

FIG. 7 is a graph of a typical waveform utilized in periodicreverse-current pulsed electroforming.

FIG. 8 is a schematic elevation view in cross-section of a section of asilicon-on-sapphire body showing one implant terminated in a rivet-likehead which makes selective contact with two circuit elements on thesilicon-faced surface of the body.

DETAILED DESCRIPTION OF THE INVENTION

The method of this invention to be described below will produce, forexample, feed-throughs useful in the transfer of signals from anintegrated circuit on one silicon-on-sapphire (SOS) wafer to one or moreintegrated circuits on the same SOS wafer and/or to one or moreintegrated circuits on different SOS wafers. Although this inventionwill be described with particular reference to silicon-on-sapphirematerial, the novel technology and objectives of this invention ofimplanting feed-through conductors is broadly applicable to othermaterials of the semiconductor arts including, for example, silicon(Si), germanium (Ge), gallium phosphide (GaP), gallium arsenide (GaAs),indium antimonide (InSb), cadmium telluride (CdTe), and zinc sulfide(ZnS).

Referring now to FIG. 1, there is shown a typical silicon-on-sapphire(SOS) body 10. Body 10 is a composite of a substrate of single crystalsapphire 11 and a contiguous overlying epitaxially-grown layer of singlecrystal silicon 12. Body 10 has front (or top) 13 and back (or bottom)14 major opposed substantially parallel surfaces and a peripheral edgearea 15 interconnecting front 13 and back 14 major surfaces. The twomajor surfaces are parallel to the (1102) plane of single crystalsapphire 11 to within ±2° and to the (100) plane of the single crystalepitaxial silicon 12 to within ±2°. The exposed silicon of the frontmajor surface 13 of wafer 10 is typically polished to an optical finishsmoother than about ±0.1 micron and the exposed sapphire of the backmajor surface 14 of the wafer 10 is typically ground to a finishsmoother than about ±0.5 micron. One or more active integrated circuitsemiconductor devices are ordinarily located in the silicon layer 12.The thickness of the epitaxial silicon layer 12 is typically less thanabout 4 microns while a typical thickness of the sapphire layer 11 is325±25 microns.

In FIG. 2, there is shown a section from the SOS body 10 of FIG. 1following the application of a conducting thin film metallic layer 16 onback major surface 14 and after the application of thin insulatinglayers 17 and 18 over the previously deposited thin film metallic layer16 and the silicon of the top major surface 13, respectively. Insulatinglayers 17 and 18 may be made using well known photolithographicaltechniques employing a photoresist or may be made from a low temperatureglass or silicon nitride and are provided to protect layer 16 andsurface 13, respectively, from damage from debris generated during thelaser drilling of holes as described below. Metallic layer 16 isrequired for the practice of this invention, but layers 17 and 18 areoptional. As a practical matter, layers 16, 17, and 18 are so thin thatthe invention may be discussed with reference to surfaces 13 and 14 orsurfaces 13 and 14 and a layer or layers of material thereoninterchangeably.

Sputtering, has proven to be the best means for depositing metal layer16 on sapphire surface 14. Other means such as chemical deposition,metal evaporation and the like do not give a metallic layer 16 with asgood adherence to sapphire surface 14 as does sputtering. The thicknessof metallic layer 16 is greater than 500 A with a preferred value of10,000 A. A suitable material for metallic layer 16 is gold. Othersuitable metals include copper, nickel, chromium and mixtures and alloysof the same.

Holes 20, as shown in FIG. 3, are drilled in SOS wafer 10 using laserbeam techniques. The novel techniques for laser drilling holes throughsilicon-on-sapphire bodies, to be described below in detail forcompleteness, are disclosed and claimed in the above-referenced commonlyassigned Ser. No. 204,957 application which is filed in the names of theinventive entity thereof, is not the invention of the inventive entityof the instant invention and does not form a part of this invention.

In schematic FIG. 3, the laser beam has impinged on bottom surface 14and layers 16 and 17 contiguous thereto and drilling has proceeded frombottom surface 14 to top surface 13; thus, entrance apertures 19 areformed in surface 14, contiguous metallic layer 16 and insulating layer17. Exit apertures 21 are formed in surface 13 and contiguous insulatinglayer 18. Interior peripheral edge area 22 interconnects entranceaperture 19 and exit apertures 21.

Since the exit aperture of the hole may be smaller than the entranceaperture, it is generally advantageous to drill from back surface 14since the exit aperture will occupy less area on surface 13 where theactive devices are located and the debris generated by drilling will beejected out the bottom. However, accuracy of location of the aperture onsurface 13 will be less than if the drilling proceeds from front surface13. Therefore, if accurate location of the feed-through on surface 13where the integrated circuits are located is the paramountconsideration, the drilling is best conducted from front surface 13 toback surface 14.

A laser is the best device for drilling the holes. Mechanical means,such as ultrasonic drilling, cannot produce holes of the fine diameterand close spacing required primarily due to drill breakage from chips inthe drill hole as attempts are made to drill holes less than about 8mils in diameter. The pulse length of other beam devices, such aselectron beams, cannot be limited to sufficiently short times to preventcracking and spalling of the wafer due to thermal stresses. By opticalinspection of the birefringent sapphire using crossed polarizers, nostrain fields were observed around the laser-drilled holes.

More particularly, a Nd-YAG laser operated in the pulsed Q-switched modeis best suited for the drilling of holes in SOS bodies. Typically, acontinuous train of laser pulses at a pulse repetition rate of 3 KHZ andan individual pulse duration of 200 nanoseconds is directed onto andperpendicular to surface 13 or 14. The continuous train of laser pulsesis beamed onto surface 13 or 14 for 5 msec and then interrupted for 45msec and repeated for 5 msec and then interrupted for 45 msec and soforth until the laser beam has drilled a hole 20 completely through body10.

Approximately 30 pulse trains are required to form holes 20 in asilicon-on-sapphire wafer that is 325 microns thick. The 3 KHZ pulserepetition rate is selected because it gives the highest output power ofthe laser operating in the repetitively Q-switched mode. Each pulsetrain is led by a giant pulse which is important because it greatlyincreases the absorption coefficient of the surface layer of body 10,allowing the following smaller pulses of the pulse train to vaporize anddrill out the material comprising body 10. A series of separate pulsetrains is used rather than one continuous pulse train in order to obtainmore of the desirable giant pulses. The delay time of 45 millisecondsbetween pulse trains is chosen so that the flash lamps surrounding theNd-YAG laser crystal have sufficient time to pump up the crystal to anenergy density where a giant pulse is produced on initiation of arepetitively Q-switched train of laser beam pulses.

A relationship between laser power level, the number of pulses requiredto drill holes 20 completely through body 10, and hole geometry andintegrity was discovered. Nd-YAG lasers with an energy rating of lessthan 1 watt in the continuous wave (CW) mode could not drill all the waythrough SOS wafers 325 microns thick. Use of higher energy lasers islimited in that if fewer than 10 pulse trains are used, cracking andspalling of the wafer 10 will occur. Lower power lasers, about 6 wattsin the CW mode, which required more than 100 pulse trains to drillthrough the 325 micron thick SOS wafers were also found to beunsatisfactory. When more than 100 pulses were required, the holes werenot straight, but exhibited a random walk effect through body 10 suchthat the exit apertures frequently were not aligned with the entranceapertures, i.e., the axes of the holes were not substantiallyperpendicular to the major surfaces. Thus the power of the Nd-YAG lasermust be such that the holes can be drilled using about 10 to about 100pulses.

At the low end of the pulse range, holes having the appearance of rightcircular cylinders with generally circular entrance and exit aperturesabout 4 mils in diameter can be produced. As the number of pulsesincreases, the diameter of the entrance aperture decreases, but theholes assume the shape of truncated right circular cones. Use of about30 pulses was found to be optimum in that truncated cone-shaped holeswith entrance diameters on the order of 2 mils and an entrance apertureto exit aperture ratio of about 2 to 1 were produced.

These holes can be spaced in arrays having center line-to-center line,i.e., axis-to-axis, spacings as small as about twice the diameter of theaperture or, in the case of holes having the shape of a truncated cone,about twice the diameter of the larger aperture.

When the Nd-YAG laser was operated in the frequency doubled mode (0.53μwavelength), at a power level sufficient to produce holes with about 30pulses, truncated cone-shaped holes having entrance aperture diametersas small as 0.5 mil and exit aperture diameters as small as 0.25 milwere produced. However, as the diameter of the aperture decreases, itbecomes more difficult to implant the conducting medium of thefeed-through conductors in the holes when conductor-bearing fluids areemployed as described in the above-referenced Ser. No. 204,957application.

It was also found that drilling, particularly from ground back surface14 of wafer 10, was facilitated by positioning a 0.025" thickpolycrystalline wafer of alumina 0.025" away from and parallel to backsurface 14 of SOS wafer 10 especially at lower power levels aproaching 6watts in the CW mode. At a separation distance greater than 0.025",drilling yields decreased. At closer separation than 0.025", there wasinsufficient distance between surface 14 and the polycrystalline aluminabackup wafer for debris from laser drilling to clear the region aroundthe intersection between holes 20 and back surface 14 of body 10 withthe result that holes 20 became clogged with drilling debris nearsurface 14.

Body 10 is ready for implantation of the feed-through conductors afterholes 20 are laser drilled therethrough. For the practice ofelectroforming in accordance with the method of this invention, body 10will typically have more than about 1000 holes therethrough since, asdisclosed in the above cross-referenced Ser. No. 285,656 application,the method of this invention is preferably and advantageously practicedwhen there are more than 1000 holes through body 10. By the termelectroforming it is meant that a solid plug (implant) of material isgrown in holes 20 under the driving force of a direct current (DC)potential. More particularly, by the method of the invention the implantis grown substantially uniformly from one major surface of the body tothe opposite major surface. Electroforming is to be distinguished fromplating which refers to the application of a thin, i.e., on the order ofseveral mils, film of metal to the surface of a material. Plating mayinclude dipping into molten metal, but it usually refers to theelectrodeposition of an adherent coating. In electroplating, metals aredeposited from solutions of their salts by means of an electric current.Electroless plating requires no externally applied current and generallyproceeds by means of chemical reduction in the presence of a catalyticmetal.

Electroforming is also to be distinguished from such other methods asthat of U.S. Pat. No. 3,483,095 wherein two solutions are reacted toproduce an ionically conductive precipitate insoluable in both solutionsand an electric field is applied to extend the precipitate through apore. A subsequent step, such as exposure to radiation or heat, isrequired to convert the ionic deposit to a conductor.

FIG. 4 shows SOS body 10 in holder 23 in tank 24. Tank 24 is equippedwith a cover (not shown) and is capable of sustaining a vacuum. Holder23, which is made from an inert material, also has means for holdinganode 25 parallel to wafer 10 and means, such as the series of holes 26,for flowing an inert gas in the space between wafer 10 and anode 25. Themajor surface of SOS wafer 10 having thin film conducting layer 16thereon is placed to face away from anode 25. Anode 25 has approximatelythe same major surface dimensions and geometry as that of wafer 10 andmay be either a solid plate or a screen of the same material, typicallycopper, to be electroformed in holes 20. Anode 25 is typically spaciallyremoved or separated from wafer 10 by about 1 cm. Gas holes 26 aretypically 1.5 mm in diameter and are typically spaced about 4 mm apart.

The positive pole of an external direct current (DC) power source 40 isconnected to anode 25 and the negative pole is connected to metal layer16. Tank 24 is then filled with enough electroforming solution (notshown) to cover anode 25 and body 10. An inert gas is slowly bubbledfrom holes 26 to the surface of the electroforming solution to providemixing and agitation of the electroforming solution.

The electroforming solution is substantially an aqueous copper platingsolution consisting essentially of from about 220 to about 270grams/liter of hydrated copper sulfate (CuSO₄.5H₂ O), from about 5 toabout 28 grams/liter sulphuric acid (H₂ SO₄), from about 0.007 to about0.013 grams/liter of thiourea (N₂ H₄ CS) and from about 0.3 to about 1.0gram/liter of molasses. The preferred solution consists essentially ofabout 250 grams/liter CuSO₄.5H₂ O, 10 grams/liter H₂ SO₄, 0.008grams/liter N₂ H₄ CS and 0.75 grams/liter molasses. It was discoveredthat baths of the above composition, of the many different compositionstried, resulted in the most even growth of copper in holes 20. With thissolution the copper growth, during the electroforming step to bedescribed, filled 100 percent of the volume of holes 20, had a brightcopper appearance, was relatively soft, and was typically greater thanabout 95% dense. The solutions would not work without molasses and ofthe several brands of molasses tried, the one marketed under the name"Brer Rabbit Molasses" proved to provide the best results. Molassescontents in excess of about 1.0 gram/liter produced an objectionablesticky residue on the wafers.

In the event that the electroforming solution does not readily wet thematerial of body 10 and, as a consequence, air bubbles are entrapped inholes 20 the following procedure may be used. First, a vacuum of about 1Torr or greater is established in tank 24. The electroformed solution isnext backfilled into tank 24 through a suitable valved opening (notshown) to cover body 10 and anode 25 while the vacuum is maintained, orreestablished if decreased during the backfilling. The vacuum ismaintained until all holes are filled and thereafter tank 24 is returnedto atmospheric pressure.

When external power source 40 is activated, an electrolytic cell isformed resulting in the deposition of copper from anode 25 onto theexposed areas of metal layer 16 adjacent to the bottoms of holes 20. Thedeposited layer will gradually build up to the point where the bottomsof holes 20 are closed by a solid bridge (not shown) of a film ofelectroformed metal thereacross.

Thereafter, implants 27 grow from the bridges at the bottoms of holes 20toward anode 25. When there are fewer than about 1000 holes 20, thegrowth rate of implants 27 in holes 20 will be substantially equal andthe implants will reach the surface facing anode 25 at about the sametime. Thus, at the end of the first stage, implants 27 will besubstantially flush with the top and bottom surfaces of body 10 and willbe substantially in the form of right circular cylinders. The diameter(D) of the cylinders will be equal to the diameter of holes 20, i.e.,will be less than or equal to about 4 mils, and will have center line30-to-center line 30 spacings (L) equal to or less than about 2D asillustrated in FIG. 5. Cylindrical implants 27 are dense, i.e., greaterthan or equal to about 95% dense, and have high-aspect-ratios, i.e.,ratio of length (thickness of body 10 as measured by the perpendiculardistance between surfaces 13 and 14) to diameter, on the order of aboutat least six-to-one. Those first-stage through-thickness implants areuseful as heat sinks or may be used for the transfer of electricalsignals by placing active devices or conductors in contact with theimplants.

Density, in the context used herein, is a measure of the compactness ofa body. The less material per unit volume, and the more volume existingas open phases within the body, the lower the density. If a body istheoretically dense, i.e., 100% dense, it contains no open spaces suchas pores or cavities. As noted above, implants 27 are also relativelysoft, i.e., have hardness on the order of that of electrolytic copper(commonly referred to as "oxygen-free" copper) which typically measuresa maximum of about 65 on the widely-recognized Rockwell R_(F) scale.

By continuing the electroforming process through the second stage,rivet-like heads 28 and 31, shown in FIG. 5, of substantially the samediameter (D_(H)) may be formed adjacent to the top and bottom surfacesor any layers thereon. Rivet-like heads 28 and 31 serve to lockfeed-through conductors 27 in place. Growth along the wafer alsofacilitates electrical contact with current leads and devices on thewafer surface. Growth of rivet-like heads 31 is due to the small leakagecurrent from the back of wafer 10 to anode 25, which is in addition tothe major currents directly from the implants 27 to anode 25.

When the growth of copper implants 27 or heads 28 and 31 reach thedesired stage, the power source is deactivated, body 10 is removed fromthe electroforming solution, washed in distilled water, rinsed inmethanol and dried.

If, however, there are more than about 1000 holes 20 per body 10, andthe growth is conducted under steady state direct current, growth in theholes will become unbalanced and certain implants will grow at a morerapid rate than the remaining implants. Thus, at the end of the firststage, the implants will not be substantially uniformly flush with thetop and bottom surfaces of the body as in the case when there are fewerthan about 1000 implants.

Further, when the fastest growing implants emerge from the holes, theirgrowth rate and the rate of growth of rivet-like heads 28 and 31 will beaccelerated, in comparison to the later-emerging implants, due toexposure to fresh electroforming solution. Although large terminations28 and 31 are desirable from a standpoint of inter-wafer contacts, largeterminations can cause problems on SOS wafers. Because of the small sizeof microelectronic circuits, larger terminations on the active surfaceof an SOS wafer may short each other and/or may also short acrosscircuit elements, e.g., elements 32 and 33 of FIG. 6, rendering theminoperative.

In order to equalize the growth rate of the implants in the holes,equalize the emergence of the implants from the holes and equalize thegrowth rate of the rivet-like terminations, the method of this inventionutilizes periodic reverse-current pulsing. As shown in FIG. 7, theelectroforming direct current is periodically (regularly) reversed togenerate a train of rectangular pulses which vary from a current levelof I_(p) in the forward, or electroforming direction for a time periodof T_(p) to a current level of I_(R) in the reverse or de-electroformingdirection for a time period of T_(R).

A net electroforming gain is achieved when

    I.sub.p T.sub.p >I.sub.R T.sub.R                           (1)

The duration of T_(R) is chosen to be less than the diffusion relaxationtime for the ion involved in the electroforming process, i.e., the ionwhich is deposited from the electroforming solution to form implant 27,in the electroforming solution in holes 20. If D_(D) is the diffusioncoefficient of the electroforming ion in the solution and H is theaverage depth of the implant beneath the top surface of body 10, thenthe criterion for selecting T_(R) is

    T.sub.R =H.sup.2 D.sub.D                                   (2)

Slower growing implants that are at a depth greater than H in a laserdrilled hole will be minimally affected by the reverse current pulse. Incontrast, faster growing implants that are closer to the surface than Hwill absorb most of the reverse current pulse and retreat backwards inthe hole. Thus the periodic reverse current pulse has a leveling effecton the height of the implants in the hole.

Using two waveform generators 41 and a controllable D.C. power supply40, a number of experiments were tried. As an example, an experimentutilizing I_(p) =-100 ma, I_(R) =+1000 ma, (electron flow convention)T_(p) =5 sec, and T_(R) =0.2 sec resulted in an even growth of copperimplants through laser drilled holes in a 13 mil thick SOS wafer. Atthese settings, the leveling control height, i.e., the height at whichperiodic reverse-current pulsing must begin, is where the implants arewithin a few mils of the emergence surface. Since T_(R) varies with H,implant growth can be controlled during the entire growth process fromthe bridges at the hole bottoms to emergence at the top surface bychanging the pulse duration T_(R) and pulse current I_(R) with time witha programmable controller.

The rate of growth of the implants using the method of this inventionwill be less than that obtained with a steady state DC power source.However, as disclosed above, the use of steady state DC results inunsatisfactory control over the time of emergence of the implants fromthe holes and the rate of growth of rivet-like heads 28 and 31 whenthere are more than about 1000 implants. Where there are less than about1000 implants and steady state DC electroforming current is employed,the diameters of the rivet-like heads D_(H) are such that for apopulation of 1,000 heads, no more than 3 heads deviate from the averagehead size by more than about +30%, i.e., the 3σ confidence limit. This3σ confidence limit is unsatisfactory where there are upwards of 10⁴ to10⁶ holes per body, of about 4 mil diameter spaced apart by about 8 mils(2D), as projected for the massively parallel information processor.Here, and in the following discussion and claims, a population ofrivet-like heads is considered to be associated with a like number ofimplants. Thus, the fact that each implant has two rivet-liketerminations is disregarded except that the population may be analyzedeither in terms of the population of a first plurality of terminationsadjacent to the top major surface of the body or in terms of thepopulation of a second plurality of terminations adjacent to theopposite, or bottom, major surface of the body.

In contrast, using the method of this invention, as illustrated by theabove-cited example, it was possible to control head diameter D_(H) sothat for a population of 1000 caps the diameter of all caps was withinabout ±3 μm of the average diameter of 200 μm. The ±3 μm variationrepresented the limits of resolution of the measuring equipmentemployed, thus ±3 μm is a worst case basis. Thus, for a population of1000 heads, no more than 3 caps will deviate from the population averageby more than about ±4.5%. For populations of 10⁴ heads, 10⁵ heads and10⁶ heads, no more than 3 heads are expected to deviate from the averagehead diameter of the population by about more than ±6%, ±7.5%, and±9.0%, respectively. These latter confidence limits are well withinthose required for the massively parallel information processor. Theforegoing may also be expressed in terms of populations of 10^(n) headswhere n is a number from 3 to 6 inclusive and no more than threeterminations (heads) in the population will be expected to deviate fromthe average diameter of the terminations in the population by more thanabout the quantity (±1.5% ×n). The actual diameters (D_(H)) of theterminations will, of course, be a function of the total electroformingtime.

To avoid the aforementioned short circuiting, but at the same timeachieve good electrical contact with the desired circuit element, themethod of this invention may be supplemented by forming a patternedinsulating layer 34 on the active side of the SOS wafer prior toelectroforming feed-through conductors 27 as shown in FIG. 8. Such aninsulating layer of low temperature glass, SiO₂, photoresist etc.,prevents the overgrowth of the feed-through termination 28 from shortingout circuit elements 33, but at the same time allows the requiredelectrical contact to aluminum landing pads 32.

Referring again now to FIG. 5, solid, dense copper feed-throughconductors 27 having rivet-like heads 28 and 31 are shown extendingbetween the major opposed surfaces of body 10. Rivet-like heads 28 and31 facilitate the stacking of one body 10 upon one or more other bodies10 thus forming a plurality of separate substantially parallelinterwafer paths along implants 27. Thus, electrical signals can bedistributed, by use of suitable switching logic (not shown), along oneor more intrawafer paths and, simultaneously if required, along one ormore interwafer paths.

Conductors 27 require no further treatment, such as curing, which may bethe case when conductor-bearing fluids are employed as described in theabove-noted Ser. No. 204,957 application. Since no curing is required,there is no possibility of shrinkage occurring and, furthermore, theseimplants are chemically and thermally stable to the projected limits ofoperation. The electrical volume resistivity of these copperfeed-throughs is typically less than 4×10⁻⁶ ohm-cm.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the true spirit and scope of theinvention as defined by the appended claims.

I claim as my invention:
 1. A process for implanting a plurality ofsolid, dense, metallic feed-through conductors comprising the stepsof:(a) providing a body, said body having top and bottom major opposedsurfaces substantially parallel to each other, an outer peripheral edgearea interconnecting said major surfaces, and a plurality of holestherethrough, one of said major surfaces being overlaid by at least athin film of a conducting metal, said metal being one selected from thegroup consisting of gold, copper, nickel, chromium, and mixtures andalloys thereof; (b) positioning said body in a tank parallel to an anodesuch that said thin film on said major surface faces away from saidanode and a space is formed between said body and said anode, the metalof said anode being the same as the metal of said solid, dense, metallicfeed-through conductors to be implanted; (c) surrounding said body andsaid anode with an electroforming solution, said electroforming solutionhaving therein at least the metal of said solid, dense, metallicfeed-through conductors to be implanted; (d) initiating a flow ofbubbles of an inert gas in said space between said body and said anode;(e) establishing a flow of direct current through said solution, betweensaid anode and said thin film; (f) forming a solid film bridge of metalacross the bottoms of said holes; (g) reversing periodically said flowof said direct current, the periodically reversed current flow beingsubstantially in the form of a periodic train of rectangular pulses,said pulses varying from a current level of I_(p) in the forwarddirection for a time period of T_(p) to a current level of I_(R) in thereverse direction for a time period of T_(R) ; and (h) growing, at asubstantially uniform rate, a plurality of solid, dense, metallicimplants from said solid film bridges at said bottoms of said holes tothe tops of said holes.
 2. The process of claim 1 wherein said pluralityof holes comprises a plurality of greater than about 1000 holes.
 3. Theprocess of claim 1 wherein the value of I_(p) is about -100 ma, thevalue of I_(R) is about +1000 ma, the value of T_(p) is about 5 seconds,and the value of T_(R) is about 0.2 second.
 4. The process of claim 1wherein the density of said implants is greater than about 95% oftheoretical.
 5. The process of claim 1 wherein said body is composed ofa single material, said single material being one selected from thegroup consisting of silicon, germanium, gallium arsenide, galliumphosphide, indium antimonide, cadmium telluride, and zinc sulfide. 6.The process of claim 1 wherein said body is a composite, said bodyhaving a single crystal sapphire substrate and a contiguous overlyingepitaxially-grown layer of single crystal silicon, said top majorsurface being the exposed surface of said silicon layer, said bottommajor surface being the exposed surface of said sapphire substrate andsaid outer peripheral edge area encompassing both said sapphiresubstrate and said layer of silicon.
 7. The process of claim 1 whereinsaid tank is capable of sustaining a vacuum.
 8. The process of claim 7further including the steps of establishing a vacuum in said tank priorto said surrounding step and, after conducting said surrounding step,maintaining said vacuum in said tank for a period of time sufficient forsaid solution to completely fill said plurality of holes and,thereafter, returning said tank to atmospheric pressure.
 9. The processof claim 8 wherein said step of establishing a vacuum comprisesestablishing a vacuum of at least about 1 Torr.
 10. The process of claim1 further including the steps of continuing the periodically reversedflow of direct current until said feed-through conductors have emergedfrom said holes and simultaneously growing the emerged feed-throughconductors away from said top and bottom surfaces and parallel to saidtop and bottom surfaces forming thereby a first plurality of rivet-liketerminations adjacent to said top surface and a second plurality ofrivet-like terminations adjacent to said bottom surface, the diametersof said rivet-like terminations being substantially equal.
 11. Theprocess of claim 10 wherein said first and second pluralities ofrivet-like terminations each comprise about 10^(n) terminations and nomore than three terminations of said first plurality or said secondplurality deviates from the average diameter of the terminations in theplurality by more than about the quantity (±1.5%×n), where n is a numberfrom 3 to 6 inclusive.
 12. The process of claim 10 wherein said firstand second pluralities of rivet-like terminations each comprise about1000 terminations, the average diameter of the terminations in saidfirst plurality and said second plurality is about 200 microns, and thediameter of each termination in each plurality is equal to or less thanabout ±3 microns of said average diameter.
 13. A process for implantingsolid, dense, copper feed-through conductors comprising the steps of:(a)providing a body, said body having top and bottom major opposed surfacessubstantially parallel to each other, an outer peripheral edge areainterconnecting said major surfaces, and a plurality of holestherethrough, one of said major surfaces being overlaid by at least athin film of a conducting metal, said metal being one selected from thegroup consisting of gold, copper, nickel, chromium, and mixtures andalloys thereof; (b) positioning said body in a tank parallel to an anodesuch that said thin film on said major surface faces away from saidanode and a space is formed between said body and said anode, the metalof said anode being copper; (c) surrounding said body and said anodewith an electroforming solution, said solution being an aqueous solutionconsisting essentially, in grams per liter, of from about 220 to about270 grams of hydrated copper sulfate, from about 5 to about 28 grams ofsulphuric acid, from about 0.007 to about 0.013 gram of thiourea, andfrom about 0.03 to about 1.0 gram of molasses; (d) initiating a flow ofbubbles of an inert gas in said space between said body and said anode;(e) establishing a flow of direct current through said solution, betweensaid anode and said thin film; (f) forming a solid film bridge of copperacross the bottoms of said holes; (g) reversing periodically said flowof said direct current, the periodically reversed current flow beingsubstantially in the form of a periodic train of rectangular pulses,said pulses varying from a current level of I_(p) in the forwarddirection for a time period of T_(p) to a current level of I_(R) in thereverse direction for a time period of T_(R) ; and (h) growing solid,dense, copper implants in said holes from said solid film bridge at thebottoms of said holes to the tops of said holes.
 14. The process ofclaim 13 wherein the electroforming solution is an aqueous solutionconsisting essentially, in grams per liter, of about 250 grams ofhydrated copper sulfate, about 10 grams of sulphuric acid, about 0.008gram of thiourea, and about 0.75 gram of molasses.
 15. The process ofclaim 13 wherein said plurality of holes comprises a plurality ofgreater than about 1000 holes.
 16. The process of claim 13 wherein thevalue of I_(p) is about -100 ma, the value of I_(R) is about +1000 ma,the value of T_(p) is about 5 seconds, and the value of T_(p) is about0.2 second.
 17. The process of claim 13 wherein the density of copperimplants is greater than about 95% of theoretical.
 18. The process ofclaim 13 wherein the resistivity of each said copper implant is lessthan about 4×10⁻⁶ ohm-cm.
 19. The process of claim 13 wherein said bodyis composed of a single material, said single material being oneselected from the group consisting of silicon, germanium, galliumarsenide, gallium phosphide, indium antimonide, cadmium telluride, andzinc sulfide.
 20. The process of claim 13 wherein said body is acomposite, said body having a single crystal sapphire substrate and acontiguous overlying epitaxially-grown layer of single crystal silicon,said top major surface being the exposed surface of said silicon layer,said bottom major surface being the exposed surface of said sapphiresubstrate and said outer peripheral edge area encompassing both saidsapphire substrate and said layer of silicon.
 21. The process of claim13 wherein said tank is capable of sustaining a vacuum.
 22. The processof claim 21 further including the steps of establishing a vacuum in saidtank prior to said surrounding step and, after conducting saidsurrounding step, maintaining said vacuum in said tank for a period oftime sufficient for said solution to completely fill said plurality ofholes and, thereafter, returning said tank to atmospheric pressure. 23.The process of claim 22 wherein said step of establishing a vacuumcomprises establishing a vacuum of at least about 1 Torr.
 24. Theprocess of claim 13 further including the steps of continuing theperiodically reversed flow of direct current until said copperfeed-through conductors have emerged from said holes and simultaneouslygrowing the emerged feed-through conductors away from said top andbottom surfaces and parallel to said top and bottom surfaces formingthereby a first plurality of copper rivet-like terminations adjacent tosaid top surface and a second plurality of copper rivet-liketerminations adjacent to said bottom surface, the diameters of saidrivet-like terminations being substantially equal.
 25. The process ofclaim 24 wherein said first and second pluralities of rivet-liketerminations each comprise about 10^(n) terminations and no more thanthree terminations of said first plurality or said second pluralitydeviates from the average diameter of the terminations in the pluralityby more than about the quantity (±1.5%×n), where n is a number 3 to 6inclusive.
 26. The process of claim 24 wherein said first and secondpluralities of rivet-like terminations each comprise about 1000terminations, the average diameter of the terminations in said firstplurality and said second plurality is about 200 microns, and thediameter of each termination in each plurality is equal to or less thanabout ±3 microns of said average diameter.